I. Field of the Invention
The present invention is directed generally to a control circuit for use with a windshield wiping system on a motor vehicle and, more particularly, to an interface circuit which facilitates the integration of a windshield moisture-sensitive wiper control system into an existing pulse-wipe windshield wiper system control which utilizes the same wiring harness as and has a circuit compatible with the existing system. Circuity configurations and algorithms in the moisture-sensitive system even permit the system to use the existing switch of the pulse-wipe system design.
II. Discussion of the Related Art
A windshield wiper control system that senses moisture and readjusts the operation of the wipers as moisture conditions change is very desirable as it obviates the need for the driver to continually readjust the wiper system manually. Systems have been developed which optically sense the presence of moisture droplets on the windshield and activate the wiper motor and adjust its speed in relation to the intensity of the precipitation encountered. An earlier patent in which the inventor of the present invention is a co-inventor, U.S. Pat. No. 4,620,141, discloses an electronic control circuit for a windshield wiper motor which includes a windshield mounted sensor module utilizing a plurality of radiant energy sources, such as LEDs, which are pulsed on and off in accordance with a pre-established duty cycle, in conjunction with a further plurality of radiant energy sensors, e.g., phototransistors, which are oriented relative to light transmitting channels such that they intersect at the outside surface of the windshield. The plural sensors are connected in a bridge configuration such that when water droplets impinge on the windshield, the light from the sources is refracted to unbalance the bridge. The signal associated with the unbalanced bridge is applied to a window comparator to produce a binary signal which is integrated and applied to a voltage-controlled oscillator that, in turn, is operatively coupled with the windshield wiper relay circuits to drive the wipers at a rate which varies as a function of the level of precipitation being encountered.
A later patent, U.S. Pat. No. 5,059,877, to the inventor in the present application also involves a control circuit for a windshield wiper system which is designed to drive the wiper blades at a rate dependent on the level of precipitation encountered but which also addresses the problem of noise associated with shifts in ambient light level. With earlier rain-responsive wiper systems, sudden or pulsating changes in the level of ambient light striking the sensor module such as, for example, those caused by shadows of utility poles or the like sweeping across the windshield, could cause false triggering and activation of the wipers independent of the presence of rain. The last-cited reference deals effectively with such interfering phenomena by using a set of sample-and-hold circuits, in conjunction with bandpass filtering and a differential amplifier under microprocessor control to linearly reject disturbances in the sensing system due to changes in ambient light, making the system immune to such disturbances.
Large motor vehicle manufacturers typically incorporate a design of wiper system designs that are common among many different vehicle models. This is true of General Motors Corporation, for example, which supplies just two basic wiper systems to cover many of its models. These are a standard wiper system which has the ability to run at either of two fixed speeds and a pulse-wipe system which has the ability to pause for a variable length of time between wipes in addition to operating at either of two fixed speeds. Both systems have mist and wash cycles and employ many common parts including the same system wiring harness.
Such a system is illustrated schematically in FIG. 1 and includes a multi-position wiper control switch 10, which is somewhat different for standard and pulse-wipe systems, but including a connector 11, wiring harness 12 with voltage supply and ground illustrated at 14 which are identical for both systems. The wiper/washer motor assembly illustrated at 16, which includes the blade parking mechanism and other linkages, are common to both systems with the exception that each employs a different control circuit board 18 which are somewhat different but also have common interface connectors with the wiper/washer motor assembly 16. From this it can readily be seen that the only actual differences between the standard and pulse-wipe systems are reduced to variations in the wiper control switch 10 and in circuit board 18, both of which can be replaced quite easily as the interconnects are the same. This degree of commonality or interchangeability makes it relatively simple for the manufacturer to provide both systems in any vehicle made and makes it relatively simple for one to upgrade from the standard to a pulse-wipe system by retrofit at the automobile dealer.
In addition to the two patents previously discussed, a variety of other references involving moisture-sensitive wiper control systems have been devised which also use various means to sense rain and control wiper action. Almost all of these systems, however, pay very little attention to integration and compatibility with existing motor vehicle systems. Such lack of compatibility and the accompanying need to provide a special wiring harness for such a system presents a serious drawback with respect to the practicality of incorporating such a system as a vehicle option by a manufacturer. It clearly would present a great advantage if such a moisture-sensitive wiper control system could be provided with the ability to be easily integrated into the scheme of a preexisting wiper system, such as the pulse-wipe system manufactured by General Motors. Such a system could readily lend itself to addition as an option at the manufacturing stage or as a retrofit on new or on the millions of existing vehicles having compatible windshield wiper control systems.